Particularly for commercial vehicles such as trucks, transmissions with a wide gear spread and a large starting gear ratio are needed. On the one hand a main driving gear is usually needed, which with the aim of optimizing fuel consumption enables a maximum permissible top speed with a low engine speed. In many cases such a driving gear is made as a direct gear in order to ensure the greatest possible efficiency. On the other hand forward-starting gears and also in particular reverse-starting gears are needed, which enable maneuvering at low speed and starting with low clutch wear. Since in addition the step intervals between the gears should not exceed a certain size, the result is a large number of gears and correspondingly greater design complexity.
An example of such a commercial vehicle transmission is the ZF AStronic transmission. This, for example, can be obtained with 12 forward gears and a starting gear ratio of 16. The transmission is essentially geometrically stepped and has between the gears step intervals in each case of 1.3, the gears being configured in three transmission groups. A first, upstream splitter group has two gears, the main transmission three gears and a downstream range group has two gears. The range group is in the form of an epicyclic or planetary transmission.
The unexamined patent application DE 101 43 994 A1 discloses a modular transmission system for multi-gear commercial vehicles, with a two-gear upstream group, a multi-gear main transmission and a multi-gear downstream group. The downstream group is in the form of a planetary transmission with a sun gear that meshes with planetary gears arranged on a planetary carrier. In addition the planetary transmission comprises a ring gear that meshes with the planetary gears and which can be connected by means of a shifting element either to the housing or to the drive output shaft of the transmission. The output shaft is connected to the planetary carrier. Thus, the downstream group as such has two gears.
The unexamined patent application DE 10 2007 010 828 A1 discloses an automated group transmission comprising a multi-gear main transmission, a splitter group connected upstream from the main transmission and a two-stage range group connected downstream from the main transmission. The upstream group has two steps and is of countershaft design. The main group has for example four steps and is also of countershaft design. The range group is a two-step planetary system wherein a sun gear is connected in a rotationally fixed manner to an extension of the main shaft on the output side and the corresponding planetary carrier is connected rotationally fixed to the output shaft of the group transmission. The ring gear is connected to a shifting packet with two shifting clutches, by means of which the range group can be shifted selectively, i.e. alternatively, between a slow-driving stage obtained by connecting the ring gear to a fixed housing component or a fast-driving stage obtained by connecting the ring gear to the main shaft or to the sun gear. Alternatively a shift connection of the ring gear to the planetary carrier is possible.
To open up further consumption-saving potentials for the future, it can be advantageous to drive in the main driving gear with a lower engine speed. This could be achieved for example by a longer geared axle. As already mentioned at the beginning, however, starting gears are also needed so that a large gear spread results. Precisely in the case of transmissions with 2-gear range groups such as the transmissions described above, this means for example increasing the stationary gear ratio of the planetary transmission. Such an increase, however, would also result in a smaller number of built-in planetaries and necessitate a relatively small sun gear, whereby with increasing spread the structural volume of the range group and the costs increase. It is also difficult to achieve a desired large starting gear ratio as the spread requirement increases.